Shotcrete is a placement method for concrete. Thus, generating the maturity curves are based on the concrete mixtures. There are several online resources about the maturity method. The Minnesota DOT provides a PDF resource that discusses production of the curves.
How much shotcrete coverage over No. 4 reinforcing bar is required?
Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.
What is the best way to check the sand-to-cement ratio in gunite batch trucks? Is it normal (common) to get up to 10% air straight from the mixing auger? I had a gunite truck fill a 5 gal. (19 L) bucket with mixed material (sand and cement) then put it in my lab mixer and wetted it up to a 3 in. (75 mm) slump and ran a test for air, unit weight, and cast a set of cylinders. I am trying to figure out the sand-to-cement ratio, but I don’t have a cement diverter to run a real-time sand-to-cement ratio test, so I am testing dispensed material to see if it makes 70% strength in 7 days, which indicates a good mixture. Are there any other ways of checking sand-to-cement proportions? I tested the mixed material in a lab about 5 minutes after it was dispensed.
By “gunite batch truck” we assume you are referring to a volumetric mixer supplying material for a dry-mix shotcrete operation. Gunite is the original tradename for what we now call dry-mix shotcrete. Here’s the description from an ACI Materials Journal (January-February 1991 issue) article about the calibration of volumetric mixers:
“To insure production of quality concrete, each volumetric-measuring unit must be calibrated for each respective concrete ingredient, following the manufacturer’s recommendations and ASTM C 685. These ingredients must be the same as those to be used in actual concrete production. The measuring devices for aggregates, cement, and dry admixtures are calibrated by weighing the discharged ingredient. Devices for water, latex modifier (if required), and liquid admixtures such as air-entraining and water-reducing admixtures generally are calibrated by weighing or measuring the volume of the discharged ingredient. The objective of calibration is to coordinate the discharge of all concrete ingredients to produce the proper mixture.”
ASTM C685/C685M states, “The proportioning and indicating devices shall be individually checked by following the equipment manufacturer’s recommendations as related to each individual concrete batching and mixing unit. Adequate standard volume measures, scales, and weights shall be made available for the checking accuracy of the proportioning mechanism.” Thus, you need to check with your equipment supplier for their recommended procedures to verify batching. Because concrete mixtures always are based on weight of ingredients there you will need to weigh a given volume to confirm the batching is accurate.
The air content test is a measure of total air so includes both entrapped and entrained air. Ten percent is definitely high. The 10% air is likely not representative of the in-place shotcrete. It may have been an issue with the lab mixer introducing more entrapped air for some reason. Estimating the air content from the unit weight test requires a good value for the theoretical unit weight. I’m not sure if you have that with the volumetric batching. You should run the air meter test (ASTM C231/C231M) to measure the air content to get a more accurate assessment.
Regarding the verification of mixture proportions, this is from ASTM C685/C685M:
“7.5 Proportioning Check—Whenever the sources or characteristics of the ingredients are changed, or the characteristics of the mixture are noted to have changed, the purchaser is permitted to require a check of the fine aggregate content and the coarse aggregate content by use of the washout test. Essentially, in the washout test, 1 ft3 [0.03 m3] of concrete is washed through a No. 4 [4.75-mm] sieve and through a No. 100 [150-µm] sieve; that retained on the No. 4 sieve is normally considered coarse aggregate whereas that passing the No. 4 and retained on the No. 100 sieve is considered fine aggregate. Corrections to the quantity of aggregates (per cubic foot or cubic meter of concrete) shall be made if the original sieve analysis of each aggregate is available.”
Because you are only interested in the sand and cement, you can simply weigh the sample of concrete, then wash out all the cement, and then weigh the remaining sand. You would need to bring the sand to roughly the same moisture content as the sand in the truck, so you aren’t including in the weight of excess water in the sand. You should note that with shotcrete impact during placement we will generally lose 50% of the air content, so your final in-place air should be around 5%. That is a reasonable value for good freezing-and-thawing durability.
I would like to better understand the limitations related to the height of install when it comes to the gunite application. Because gunite is a dry concrete mixed with water at the nozzle of the applying apparatus, I have been told by others in the industry that the application is only intended for use of walls less than 4 or 5 ft tall. If that is the case, is it safe to assume that the gunite application strategy should not be used for below-grade vaults exceeding a height of 5 ft? I am looking for design literature specific to gunite.
Dry-mix shotcrete adds mixing water to the dry concrete materials as the concrete materials flow through and out the nozzle. Gunite is the original tradename for dry-mix shotcrete. Though you may not find design information using the old gunite name, you will find numerous current design references to dry-mix shotcrete. This includes ACI 506R-16, “Guide to Shotcrete”; ACI 506.2-13, “Specification for Shotcrete”; ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote”; ACI 372, “Design and Construction of Circular Wrapped Prestressed Concrete Tanks”; ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures”; ACI 350.5, “Specifications for Environmental Concrete Structures”; and seven ASTMs that directly cover shotcrete. ACI 318-19, “Building Code Requirements for Structural Concrete,” has also added specific shotcrete provisions. Dry-mix shotcrete has been used for decades to build structural concrete walls over 50 ft high in circular prestressed concrete tanks that withstand a full head of water pressure. This is a substantially greater water pressure than your 5 ft vault wall would experience. There are no limitations in the dry-mix placement process that would preclude use in high walls. Both dry-mix and wet-mix shotcrete using quality materials, proper equipment, and experienced placement crews will produce in-place concrete of equal strength, durability, and low permeability. However, generally wet-mix shotcrete can offer placement rates up to four times higher than dry-mix. Thus, in thicker, longer walls, wet-mix shotcrete may be more cost-effective because it can be placed faster.
One quick question regarding the “grading” of cores based on 506.2: I imagine the 1995 standard-based 1 through 5 grading system is no longer valid? In one case the special inspector has failed a core (score of 5) simply because a piece of it broke off during the coring and removal from the coring cylinder. I’m leery of that type of failed rating. What do you think about that, and core grading in general?
The current version of the ACI “Specification for Shotcrete” is ACI 506.2-13 and purposely eliminated the core grading due to many problems that arose in the field in trying to equitably apply the procedure. ACI 506 has a new document ACI 506.6T-17, “Visual Shotcrete Core Quality Evaluation Technote,” that is intended to assist specifiers with evaluating shotcrete core quality for an intended use. So yes, the 506.2-95 version of the specification and the grading system it included is no longer supported as an industry standard.
Regarding grading, a shotcrete score of 5 under the deprecated grading system just because the core broke during coring is unreasonable. The coring operation exerts a massive force on the core, as it basically rips it out of the concrete. We have seen many instances where the core snapped where a large reinforcing bar horizontally crossed the core and effectively created a slip plane that reduced the shear resistance of the core section to the torque created by the coring. In those cases, examining the core hole for any evidence of shotcrete problems would be appropriate and then discounting the core itself.
In summary, my position and the ACI 506 Committee’s position is that core grading should not be used for evaluating shotcrete quality. The ACI 506.6T-17 Technote document should be the current reference for making a reasonable evaluation of shotcrete quality for the intended use.
What is the cure time for shotcrete before paint can be sprayed? And what type of paint (latex or oil-based) would work best?
Shotcrete is a placement method for concrete. Thus, cure time is the same as what the coating manufacturer recommends for new concrete surfaces. Because moisture in the concrete surface is part of the concern of bond from manufacturers, a water-based latex may be preferable to oil-based, but you should discuss the appropriate time and materials with your coating supplier.
We are applying a new shotcrete shell to an existing building. We are calling for a 5 in. (125 mm) base coat with a 1/4 in. (6 mm) brush-finished flash coat. We are specifying a color admixture for all of the shotcrete. It is a large building and we anticipate several days of shotcrete operations. Would it be wise to allow (or even specify): 1) the color admixture to be used in the flash coat only? 2) all of the flash coat to be applied at once, possibly days after all of the base coat has been installed, to achieve a more uniform final appearance?
If you only need color in the exposed shotcrete surface, just putting color in the flash coat should be adequate as long as the flash coat is continuous across the entire area. You may want to specify the final “flash” layer to be thicker, perhaps 1/2 in. (12 mm) to 1 in. (25 mm) thick. This would help assure consistent color and still allow the finishers to produce a consistent final texture. As long as the concrete materials are the same in the production, you wouldn’t need to shoot the final layer all in one day. Also, be sure curing methods are the same for the entire area as varying moisture can sometimes affect the early appearance of the coloring, though it usually balances out over time.
I am currently researching shotcrete machines for a sewerage channel. Would dry-process or wet-process be more functional and efficient? I am new to the industry, so could you please give me some background information regarding the equipment used as well as information about the techniques and processes you would use? Could you also inform me about the factors like humidity and what effect they would have on what process you would use and how you would apply it? Please also let me know about any other information that you would consider as having a significant effect on the overall process.
The thickness of the shotcrete placement, site logistics, and the availability of ready-mixed concrete, as well as expertise of the shotcrete contractor would likely determine the most cost-effective method for shotcrete placement. Both dry-mix and wet-mix will produce structural concrete in place with similar physical properties and durability. Wet-mix can generally produce more volume of material placed per hour than dry-mix. You had several basic questions about shotcrete that can be answered by visiting sections of Shotcrete.org. On our website, you will find the informational pages shotcrete.org/Resources and shotcrete.org/ArchiveSearch beneficial. The Resources page lists many shotcrete-specific articles and web resources. The Archive Search allows you to search through our Shotcrete magazine archive for past articles using keywords. You may also find ACI 506R-16, “Guide to Shotcrete,” an informative primer on all aspects of shotcrete. You can purchase a PDF copy of the Guide from ACI at www.concrete.org/store/productdetail.aspx?ItemID=50616. ACI Committee 506, Shotcreting, also has several other technical documents available for specification for shotcrete, evaluation of shotcrete cores, specifying underground shotcrete, and fiber-reinforced shotcrete you can find on the ACI website.
ACI 506R-16 discusses surface preparation requirements for various substrate surfaces and notes that for earth surfaces shotcrete shall not be placed on frozen ground. There does not appear to be any specific temperature requirement for other substrate materials, however. For concrete or masonry sub-straight surfaces, are there temperature requirements for shotcrete application in situations where bond is not required?
All surfaces receiving shotcrete should be above freezing. The mandatory requirements of ACI 506.2-13, “Specification for Shotcrete,” specifies:
“3.4.5 Cold weather shotcreting—Unless otherwise specified, shooting may proceed when ambient temperature is 40°F and rising. Stop shooting when ambient temperature is 40°F and falling, unless measures are taken to protect the shotcrete. Shotcrete material temperature, when shot, shall not be less than 50ºF. Do not place shotcrete against frozen surfaces.”
Your question then asks about substrate temperatures for sections not requiring bond. The concern of frozen concrete is not only bond, but an issue with freezing of some thickness of the concrete that would prevent strength gain. For requirements on this, ACI 301-16, “Specifications for Structural Concrete,” would likely apply and 5.3.2.1(b) requires:
“5.3.2.1(b) Cold weather—Concrete temperatures at delivery shall meet the requirements of 4.2.2.5. Do not place concrete in contact with surfaces less than 35°F. Unless otherwise specified, this requirement shall not apply to reinforcing steel.”
There is a discrepancy between ACI 301 and ACI 506.2. The ACI 301 value (35°F) is somewhat more conservative, though ACI 506.2 provisions (32°F) have proven to produce quality shotcrete. You may consider asking the Engineer of Record for your project what minimum substrate temperature is acceptable on your specific job.
How much shotcrete coverage is required over No. 4 reinforcing bar?
Shotcrete is simply a placement method for concrete. The specified concrete cover over reinforcing bar is usually included in contract documents for construction and values vary depending on exposure conditions. ACI 318 provides cover requirements for structural concrete in buildings, and ACI 350 provides cover requirements for concrete liquid-containing structures. Local building codes and fire codes may also require specific cover in concrete construction. If your project doesn’t specify the cover requirements, we recommend you consult with a professional engineer experienced in the type of project you are working on to learn what the code requirements may be.